1. Field of the Invention
The present invention relates to an apparatus and method for in-furnace reduction of nitrogen oxide emissions in flue gas.
In the combustion of fuels with fixed nitrogen such as coal, oxygen from the air may combine with the fixed nitrogen to produce nitrogen oxides. At sufficiently high temperatures, oxygen reacts with atmospheric nitrogen to form nitrogen oxides. Production of nitrogen oxide is regarded as undesirable. There are numerous government regulations which limit the amount of nitrogen oxide which may be emitted from a combustion furnace. Furthermore, the presence of nitrogen oxide in a furnace flue gas causes the condensates formed from the gases to be more corrosive and acidic. Consequently, there is a need for apparatus and processes which reduce the nitrogen oxide emissions in furnace flue gas.
2. Description of the Prior Art
Numerous attempts have been made to develop apparatus and processes which reduce the nitrogen oxide emissions in a furnace flue gas. One such approach is a process known as in-furnace nitrogen oxide reduction, reburning, or fuel staging. In reburning, coal, oil, or natural gas is injected above the normal flame zone to form a fuel-rich zone. In this zone, part of the nitrogen oxides are reduced to ammonia- and cyanide-like fragments which are then oxidized to form N.sub.2 and nitrogen oxide.
Several problems occur when this process is used. First, coal may be an inefficient reburn fuel because of its high fixed-nitrogen composition. The fixed nitrogen introduced at this location in the furnace will have less chance of being converted to N.sub.2, and therefore have a higher chance of ending up as nitrogen oxide and may, depending on the nitrogen oxide concentration of the flue gas, increase the emissions of nitrogen oxide.
Furthermore, the fuel must be injected with a sufficient volume of gas. If air is used as this gas, there must be enough fuel to consume the oxygen in the flue gas and air, and to supply an excess of fuel so reducing conditions exist. This increases the amount of fuel which must be used as reburn fuel. Furthermore, the necessity of using carrier air requires extensive duct work in the upper part of the furnace.
Additionally, the reburn fuel must be injected well above the primary combustion zone of the furnace so that it will not interfere with the reactions taking place therein. However, this fuel must be made to burn out completely without leaving a large amount of unburned carbon. To do this, the fuel must be injected in a very hot region of the furnace some distance from the furnace exit. The exit temperature of the furnace must be limited in order to preserve the heat exchanger's surface. Therefore, a tall furnace is required to complete this second stage process.
Moreover, the fuel must be injected in such quantities as to make the upper furnace zone fuel rich. This fuel is supplied in excess of the amount of air in the furnace and ultimately requires more air in order to be completely combusted. Thus, air must be injected above the reburn fuel injection. This requires even more duct work and furnace volume.
Finally, most coal furnaces which are now in operation are not designed to accommodate the prior art methods. Major modifications such as the provision of extensive duct work and the addition of a second stage to the process are required to utilize the prior art method. Such retrofitting is expensive. Consequently, there is a need for a combustion apparatus and process which will reduce nitrogen oxide emissions in flue gas and which can be readily used in existing furnaces.
In U.S. Pat. No. 4,779,545, a reburn process wherein natural gas is introduced in the upper furnace through pulse combustors is described. This process does not require any make-up air to reduce NO.sub.x emissions. However, it does require pulse combustors, the installation and operation of which is costly. There remains a need for an improved process for the in-furnace reduction of nitrogen oxides.
In U.S. Pat. No. 4,779,545, the inventors teach that nitrogen oxide emissions can be reduced by introducing fuel into the upper furnace in discrete pulses which generate fuel-rich pockets. In these zones, part of the nitrogen oxide is reduced to N.sub.2 and part to NH.sub.3, NH.sub.i, and other reduced nitrogen species. As the fuel-rich zones mix with the remainder of the flue gas, the reduced nitrogen species react with more nitrogen oxides to form N.sub.2.
The process of U.S. Pat. No. 4,779,545 requires sufficient time for the formation of fuel-rich pockets, the mixing and chemical reactions within the pockets, and the out mixing followed by the next set of reactions. Careful control or selection of chemical ratios, temperatures, times, and turbulence is required.
Our invention depends only upon the diffusion of the fuel into the combustion products. A two-step process is not required.